Hello, os_send_message(unsigned char mailbox, unsigned int message, unsigned char timeout) The RTX51 manual page 76 stated about the parameters; message: is the message to be sent. mailbox: is the identification!!! of the mailbox !!!!!. Permissible values are 0..7. Also stated" A mailbox can store a maximum of 8 messages". The question is if I substitute for the parameter mailbox by 1. does 1 mean mailbox 1 (2nd mailbox) and this mailbox can store 8 messages, or the parameter mailbox can be 0..7, means these are the 8 messages. I assume that, the mailbox paramter is the message number in the mailbox and there is only one mailbox. Am I correct? Thanks